Apparatus and methods for enhancing the detection of small-source plumes from moving aircraft

ABSTRACT

Detection of small-source plumes, as by an airborne instrument, wherein ions formed from the molecules of a gaseous sample and collected by the airborne instrument are segregated in a drift field, and signals produced in response to the detection of the segregated ions are separated into short-duration plume signal components and long-duration background components. The shortduration components are indicated with enhanced resolution.

United States Patent Wernlund 51 Oct. 10, 1972 [54] APPARATUS AND METHODS FOR ENHANCING THE DETECTION OF SMALL-SOURCE PLUMES FROM 250/419 TF, 43.5 R, 43.5 FC

[56] References Cited UNITED STATES PATENTS 2,463,544 3/1949 Langmeier ..250/4l.9 3,254,209 5/1966 Fite ..250/41.9

Primary Examiner-Archie R. Borchelt Assistant Examiner-C. E. Church Attorney-Raphael Semmes [5 7] ABSTRACT Detection of small-source plumes, as by an airborne instrument, wherein ions formed from the molecules of a gaseous sample and collected by the airborne instrument are segregated in a drift field, and signals produced in response to the detection of the segregated ions are separated into short-duration plume signal components and long-duration background components. The short-duration components are indicated with enhanced resolution.

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' FILTER APPARATUS AND METHODS FOR ENHANCING THE DETECTION OF SMALL-SOURCE PLUMES FROM MOVING AIRCRAFT BACKGROUND OF THE INVENTION This invention relates to the detection of smallsource plumes from moving vehicles and is more particularly concerned with emphasizing short-duration signal components in the presence of long-duration 1 discloses Plasma Chromatography systems involving the formation of primary ions and reaction of the primary ions with molecules of trace substances to form secondary ions, which may be concentrated, separated, detected, and measured by virtue of the difference of the Velocity or mobility of the ions in an electric drift field. Primary ions produced by subjecting the molecules of a suitable host gas, such as air, to ionizing radiation, for example, are subjected to the drift field, causing them to migrate in a predetermined direction through a reaction space into which the sample or trace gas is introduced. The resultant collisions between the primary ions and the trace gas molecules produce secondary ions of the trace gas in much greater numbers than can be produced by mere electron attachment to trace gas molecules. The secondary ions are also Subjected to the drift field and the ions are sorted in accordance with their velocity or mobility. A specific system of the co-pending application employs a pair of successively arranged shutter grids or ion gates for segregating the ion species in accordance with their drift time. The opening of the first gate is timed to pass a group of ions, which may comprise unreacted primary ions as well as secondary ions, and the opening Of the second gate is timed to pass a portion of the group to an ion detection means.

The Plasma Chromatograph has the ability to measure very small concentrations of trace materials in gaseous samples (eg mole fraction) very rapidly (in seconds) and to perform such measurements at atmospheric pressure. In fact, optimum operation of the Plasma Chromatograph requires that the pressure be maintained at a level which will ensure that the length of the mean free path of the ions is very much less than the dimensions of the ion formation and drift space, so that the number of ion collisions is very high and so that the ions reach a terminal velocity in the drift space.

One of the applications of the Plasma Chromatograph is in the detection by a vehicle-carried instrument Of gaseous traces or plumes in the atmosphere, such as vehicular exhaust or products of combustion. For such use, the Plasma Chromatograph must be able to produce a well-defined response to a smallsource plume when the instrument (more specifically its sample inlet) passes through the plume. The instrument response produced by a pass through such a plume will be a rapid on-off signal with a duration of the order of a few seconds. The instrument must be able to resolve such a short-duration signal from a longduration background which tends to mask the short-duration signal. Although the invention will be described in connection with an airborne instrument, land or water transportation may also be employed.

BRIEF DESCRIPTION OF THE INVENTION It is accordingly a principal object of the present invention to provide improved apparatus and methods for detecting short-duration signals in the presence of long-duration background signals in a Plasma Chro- 0 matograph measurement system.

Briefly stated, the present invention involves the separation of the output signals of a Plasma Chromatograph into short-duration and long-duration components by frequency filtration techniques, the suppression of the long-duration components, and the enhancement of the short-duration components.

BRIEF DESCRIPTION OF THE DRAWING The invention will be further described in conjunction with the accompanying drawing, which illustrates preferred and exemplary embodiments, and wherein:

FIG. 1 is a combined block and schematic diagram of a first embodiment of the invention; and

FIG. 2 is a block diagram of a second embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION The present invention is concerned with Plasma Chromatography, a trace gas measurement technique which employs an instrument having the characteristics set forth above, such as the instruments set forth in the aforesaid co-pending Application Ser. No. 777,964 or in other co-pending applications, such as Ser. No. 780,851 now Pat. 3,626,180, filed Dec. 3, 1968; Ser. No. 790,108, filed Jan. 9, 1969; or Ser. No. 828,402, filed May 27, 1969 now Pat. No. 3,621,240. As shown diagrammatically in FIG. 1, the Plasma Chromatograph instrument employs a cell 10 including an envelope E containing a pair of principal electrodes C and A adjacent to the opposite ends of the envelope and a pair of shutter grids G1 and G2 adjacent to the principal electrodes respectively. Electrode C is provided with or associated with an ionizer, such as a tritium foil supported on the electrode. A drift field is provided between the electrodes C and A by a D.C. source S, and guard rings R connected to a suitable voltage divider (not shown) across the source maintain the uniformity of the electric drift field.

Ions of trace molecules in a sample admitted to the envelope by an inlet 1 (which may be a scoop exposed exteriorly of an aircraft) are formed by ion-molecule reactions between the trace molecules and reactant ions. The reactant ions may be produced by the ionizer from reactant gas molecules in the sample or from a separate source. For example, if the sample is air, the reactant ions may be oxygen or nitrogen ions from the sample. The drift field applied between the principal electrodes C and A causes the ions to drift along the length of the chamber from C to A, the polarity of the source S being dependent upon the polarity of the ions to be detected. The pressure in the envelope E is maintained at a level, preferably atmospheric, which ensures that the length of the mean free path of the ions is very small compared to the dimensions of the ion formation and drift space.

upon the delay interval between the opening of the grids. By varying the time at which grid G2 is opened, the entire ion drift time spectrummay be produced in the output.

A non-reactive drift gas, such as nitrogen, may be admitted to the envelope through inlet I, so as to fill substantially the entire drift space between G1 and G2 and suppress ion-molecule reactions in this region, ionmolecule reactions thus being restricted to the region adjacent to electrode C. An exhaust outlet is provided for the removal of the sample and the drift gas. in the case of an airborne instrument, the sample may be rammed into theinlet and out of the exhaust by the movement of the aircraft through the air.

It has been discovered that when Plasma Chromatograph measurements are performed from a moving aircraft, the short-duration plume signals to be detected may be masked by long-duration background signals. During a sampling interval, the aircraft-carried Plasma Chromatograph may experience a rise in the background signal output (due to trace substances dispersed in the atmosphere) to a level which will persist for a length of time which may be many times longer than the signal experienced in passing through a plume. Thus, as shown in FIG. 1, the electrical output from the -Plasma Chromatograph cell may comprise a short-duration plume signal P riding on a longduration background signal B. It has been discovered that the short-duration plume signal can be differentiated from the background signal and emphasized by methods which will nowbe described.

As shown in FIG. 1, the output from the Plasma Chromatograph cell 10 is applied to a low-pass filter 14 of appropriate time constant, which blocks the short-duration plume signal P and passes the background signal B. The background signal component may then be applied to the read-out apparatus 16 for suppressing the long-duration background signal component in the output of the cell 10 and permitting only the short-duration signal component to be recorded by a recorder 18. Thus, the signal shown applied to the recorder 18 comprises the plume signal P without the background signal.

The suppression of the background signal may be accomplished, for example, by utilizing the background component B supplied from the low-pass filter 14 to operate the automatic gain control AGC of the readout electronics, which may include an electrometer having an amplifier with a conventional automatic gain control. The gain may be adjusted so that only the peaked plume signal has sufficient amplitude to be passed by the read-out electronics to the recorder. Thus, in effect, the base level of the recorder or indicator is set so that only plume signals of short-duration are recorded. In some instances, the background componentB may be applied directly to a zero adjusting element of the recorder (or indicating meter), and the recorder base line may thus be adjusted to zero in the absence of the desired plume signal.

The invention takes advantage of the fact that the short-duration plume signal and the long-duration background signal have different characteristic Fourier component frequencies, which may be separated by filters of appropriate frequency response. The low-pass filter of FIG. 1 may have a band-pass of from zero to 0.08 Hz. The upper limit may be as high as 1.6Hz or as low as 0.001 Hz.

FIG. 2 illustrates an embodiment of the invention in which the short-duration plume signal is emphasized by passing the output of the Plasma Chromatograph cell 10 through a high-pass filter of appropriate time constant, such as a simple capacitor 20. The high-pass filter may be a portion of the read-out electronics or interposed between block 16 and the recorder 18, as shown. A suitable comer frequency or lower limit for the highpass filter is from 0.001 to 0.08 Hz.

While preferred embodimentsof the invention have been shown and described, it will be apparent to those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which-is defined in the appended claims. For example, the systems of the invention may incorporate multiple low-pass filter sections (as in the read-out apparatus) for attenuating undesirable high-frequency noise from all sources, while passing the desired (relatively lower) signal frequencies.

The invention claimed is:

1. A method of testing a gaseous sample with enhanced resolution, which comprises providing during a sampling period a gaseous sample having a long-duration component and a short-duration component, ionizing molecules of said sample, including molecules of said components, producing an electrical signal in response to the ions, including a background signal component of long-duration and a trace signal component of relatively short-duration, and separating said signal components.

2. A method in accordance with claim l, wherein said signal components are separated by passing a portion of said signal through a low-pass filter.

3. A method in accordance with claim 2, wherein said signal is applied to signal indicating means and wherein the base indicating level of said signal indicating means is varied in accordance with the output of said low-pass filter.

4. A method in accordance with claim 1, wherein said signal components are separated by applying said signal to a high-pass filter, and wherein the output of said filter is indicated.

5. A method in accordance with claim 1, wherein the ionizing of molecules of said sample comprises reacting molecules of said sample with primary ions to produce secondary ions.

6. A method in accordance with claim 1, wherein said ions are segregated in accordance with their velocity in a drift field and said signal is produced in response to the detection of at least some of the segregated ions.

7. A method in accordance with claim 6, wherein said ionizing and segregating occurs in a region the dimensions of which are large compared to the length of the mean free path of said ions in said region.

8. A method in accordancd with claim 1, wherein the method is performed by collecting a sample aerially.

9. A method in accordance with claim 1, wherein the method is performed by collecting a sample of a plume by moving a vehicle past the plume.

10. An apparatus for testing a gaseous sample with enhanced resolution, which comprises means for providing, during a sampling period, a gaseous sample having a long-duration component and a short-duration component, means for ionizing molecules of said sample, including molecules of said components, means for producing an electrical signal in response to said ions, including a background signal component of long duration and a trace signal component of relatively shortduration, and means for separating said signal components.

11. Apparatus in accordance with claim 10, wherein said separating means comprises a low-pass filter.

12. Apparatus in accordance with claim 11, further comprising indicating means, and means responsive to the output of said low-pass filter for controlling the base indicating level of said indicating means.

13. Apparatus in accordance with claim 10, wherein said separating means comprises a high-pass filter.

14. Apparatus in accordance with claim 10, wherein said ionizing means comprises means for reacting molecules of said sample with primary ions to produce secondary ions.

15. Apparatus in accordance with claim 14, further comprising means for subjecting said ions to a drift field and for segregating said ions in accordance with their velocity in said field, and wherein said signal producing means is responsive to at least some of the segregated ions.

16. Apparatus in accordance with claim 15, said means for ionizing said molecules and segregating said ions comprising a drift cell, and means for maintaining the length of the mean free path of ions in said drift cell very small compared to the dimensions of said cell.

17. Apparatus in accordance with claim 10, wherein said separating means comprises a low-pass filter having an upper limit of the order of 1.6 Hz or less.

18. Apparatus in accordance with claim 10, wherein said separating means comprises a high-pass filter having a lower limit of the order of 0.08 Hz or less.

19. Apparatus in accordance with claim 10, wherein said apparatus includes means for attenuating noise 

1. A method of testing a gaseous sample with enhanced resolution, which comprises providing during a sampling period a gaseous sample having a long-duration component and a shortduration component, ionizing molecules of said sample, including molecules of said components, producing an electrical signal in response to the ions, including a background signal component of long-duration and a trace signal component of relatively shortduration, and separating said signal components.
 2. A method in accordance with claim 1, wherein said signal components are separated by passing a portion of said signal through a low-pass filter.
 3. A method in accordance with claim 2, wherein said signal is applied to signal indicating means and wherein the base indicating level of said signal indicating means is varied in accordance with the output of said low-pass filter.
 4. A method in accordance with claim 1, wherein said signal components are separated by applying said signal to a high-pass filter, and wherein the output of said filter is indicated.
 5. A method in accordance with claim 1, wherein the ionizing of molecules of said sample comprises reacting molecules of said sample with primary ions to produce secondary ions.
 6. A method in accordance with claim 1, wherein said ions are segregated in accordance with their velocity in a drift field and said signal is produced in response to the detection of at least some of the segregated ions.
 7. A method in accordance with claim 6, wherein said ionizing and segregating occurs in a region the dimensions of which are large compared to the length of the mean free path of said ions in said region.
 8. A method in accordancd with claim 1, wherein the method is performed by collecting a sample aerially.
 9. A method in accordance with claim 1, wherein the method is performed by collecting a sample of a plume by moving a vehicle past the plume.
 10. An apparatus for testing a gaseous sample with enhanced resolution, which comprises means for providing, during a sampling period, a gaseous sample having a long-duration component and a short-duration component, means for ionizing molecules of said sample, including molecules of said components, means for producing an electrical signal in response to said ions, including a background signal component of long duration and a trace signal component of relatively short-duration, and means for separating said signal components.
 11. Apparatus in accordance with claim 10, wherein said separating means comprises a low-pass filter.
 12. Apparatus in accordance with claim 11, further comprising indicating means, and means responsive to the output of said low-pass filter for controlling the base indicating level of said indicating means.
 13. Apparatus in accordance with claim 10, wherein said separating means comprises a high-pass filter.
 14. Apparatus in accordance with claim 10, wherein said ionizing means comprises means for reacting molecules of said sample with primary ions to produce secondary ions.
 15. Apparatus in accordance with claim 14, further comprising means for subjecting said ions to a drift field and for segregating said ions in accordance with their velocity in said field, and wherein said signal producing means is responsive to at least some of the segregated ions.
 16. Apparatus in accordance with claim 15, said meAns for ionizing said molecules and segregating said ions comprising a drift cell, and means for maintaining the length of the mean free path of ions in said drift cell very small compared to the dimensions of said cell.
 17. Apparatus in accordance with claim 10, wherein said separating means comprises a low-pass filter having an upper limit of the order of 1.6 Hz or less.
 18. Apparatus in accordance with claim 10, wherein said separating means comprises a high-pass filter having a lower limit of the order of 0.08 Hz or less.
 19. Apparatus in accordance with claim 10, wherein said apparatus includes means for attenuating noise frequencies higher than said trace signal component. 